Pregnancy Due Date Calculator
Estimate your due date from your last menstrual period using Naegele's rule — Due Date = LMP + 280 days, adjusted for cycle length. Solve for due date, LMP, or conception date, and see your full 40-week pregnancy timeline with milestone dates, trimester tracking, and clinical reference tables.
Only about 4% of babies arrive on their EDD
Naegele's rule gives the midpoint of a wide distribution, not a prediction of the exact date. In a landmark study of nearly 500,000 spontaneous births, only 4.4% occurred on the calculated due date. Roughly 60% fell within one week either side, and 90% within two weeks. What this means in practice: a due date is not an appointment, it is the centre of a range. Clinicians use it to schedule monitoring, decide when to intervene, and monitor growth, but the actual birth date is largely determined by factors the formula cannot see.
The 28-day assumption is why cycle length matters
Naegele's rule was derived in an era when the average cycle was assumed to be exactly 28 days with ovulation on day 14. That is true for only about half of women. A woman with a 35-day cycle ovulates around day 21, so her due date is about 7 days later than the rule would suggest. A woman with a 21-day cycle ovulates around day 7, so her due date is 7 days earlier. The cycle-length adjustment in this calculator handles this: EDD = LMP + 280 + (cycle − 28). Without this adjustment, cycle length alone can shift the EDD by a full week.
First-trimester ultrasound is more accurate
ACOG recommends confirming the estimated due date with an early ultrasound when available. A crown–rump length measurement between 7 and 14 weeks estimates the due date to ±5–7 days — more accurate than any LMP-based rule when the LMP date is uncertain, when cycles are irregular, or when the patient conceived on hormonal contraception. Second-trimester ultrasound (±10–14 days) is less precise than first-trimester, and third-trimester ultrasound (±21–30 days) should not be used for dating at all. When ultrasound and LMP disagree by more than 7 days in the first trimester, the ultrasound date is typically adopted.
Post-term pregnancy is defined by the EDD
A pregnancy that reaches 42 weeks 0 days (294 days) without delivery is considered post-term, and the risk of stillbirth, meconium aspiration, and maternal complications rises after this point. Most obstetric guidelines recommend induction by 41–42 weeks, or earlier depending on risk factors. Accurate dating is therefore clinically important — an EDD that is 7 days too early can lead to unnecessary early induction, while an EDD that is 7 days too late can allow a post-term pregnancy to develop. This is another reason early ultrasound is preferred when available.
How to Use This Calculator
Naegele's rule is a simple date addition: EDD = LMP + 280 days, adjusted for cycle length. Pick which variable you want to solve for, enter the values you know, and read the full 40-week timeline.
1. Choose what to solve for
Select EDD, LMP, or Conception from the toggle. If you know your LMP, solve for EDD. If you know the due date given by your clinician, solve for LMP. If you know the conception date (e.g. from IVF), solve for EDD.
2. Set your cycle length
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles are regularly longer or shorter, adjust the cycle length. Each 1-day deviation shifts the due date by 1 day. If your cycles are irregular, use an early ultrasound instead.
3. Enter the LMP date
Use the first day of bleeding of your most recent period — not the last day. If you are unsure or your cycles are irregular, the calculator will still work but the result is less reliable than an ultrasound-based date.
4. Read the result and current status
The result card shows your estimated due date plus your current gestational age, trimester, and days remaining. The progress bar below shows where you are on the 40-week timeline, and the milestone strip shows the week numbers of key pregnancy events.
5. Check the timeline and milestones
The schematic shows the full 42-week timeline with trimester bands, your current position, and key dates. In advanced mode, a milestone chart lists the actual calendar dates for conception, end of first trimester, viability, term, and more.
6. Switch to Advanced for depth
Advanced mode adds a milestone calendar with actual dates, a chart showing how the due date shifts with cycle length, a sensitivity panel, and companion obstetric metrics including conception date, trimester end dates, and clinical milestone references.
What This Tool Offers
Built around Naegele's rule — EDD = LMP + 280 days — with cycle-length adjustment and a complete 40-week pregnancy timeline.
Three-way solving
Solve for estimated due date, last menstrual period, or conception date. The calculator rearranges Naegele's rule for whichever variable you choose.
Live pregnancy timeline
A 42-week horizontal timeline with trimester bands, your current position marked, and key milestone dates plotted below.
Trimester verdict
Every result is classified by trimester — first, second, or third — with a colour-coded badge and a progress bar showing exactly where you are on the 40-week timeline.
Milestone calendar
A five-chip strip on the result card and a full milestone chart in advanced mode showing the calendar dates for end of first trimester, viability, term, EDD, and more.
Four coordinated charts
A 42-week timeline, trimester progress bars, a milestone calendar, and a due-date vs. cycle-length chart — each annotated with your current operating point.
Clinical reference tables
A gestational age reference table with fetal development milestones, a due-date lookup grid by LMP, and a complete variable reference with clinical context.
Reference Tables & Conversions
A gestational age calculator, a due-date lookup grid, and the full pregnancy milestone reference table.
Gestational age calculator — LMP to GA
Due Date Lookup Grid
Estimated due date by LMP date, assuming a standard 28-day cycle. Values shown as month/day in the following year.
| LMP month | LMP day 1 | LMP day 8 | LMP day 15 | LMP day 22 | LMP day 28 |
|---|
Pregnancy Milestone Reference
Key gestational weeks with fetal development, clinical significance, and the corresponding calendar date for your LMP.
| Week | Milestone | Development / Clinical | Your date |
|---|
Variable Reference
| Symbol | Meaning | Typical range | Notes |
|---|---|---|---|
| LMP | Last menstrual period | 0 – 42 weeks ago | First day of most recent period |
| EDD | Estimated due date | — | LMP + 280 days + cycle adjustment |
| GA | Gestational age | 0w 0d – 42w 0d | Weeks + days since LMP |
| Cycle | Menstrual cycle length | 21 – 35 days | Adjusts for ovulation timing |
| Conception | Estimated fertilisation | — | LMP + (cycle − 14) days |
| T1 | First trimester | 0w – 13w 6d | Major organ formation |
| T2 | Second trimester | 14w – 27w 6d | Growth and movement |
| T3 | Third trimester | 28w – 42w | Fetal maturation and delivery prep |
| Term | Full term | 39w 0d – 40w 6d | ACOG definition of optimal delivery |
| Viability | Age of potential survival | 24w 0d | With intensive neonatal care |
What Does Naegele's Rule Measure?
Naegele's rule estimates the expected date of delivery from the first day of the last menstrual period. Published by German obstetrician Franz Naegele in the early 19th century, it assumes a 28-day menstrual cycle with ovulation on day 14, and a total gestation of 280 days (40 weeks) from LMP to birth. Despite its age and simplicity, it remains the standard method used by clinicians worldwide — adjusted for cycle length, and confirmed by early ultrasound when available. It gives the midpoint of a distribution, not a prediction, but that midpoint is what matters for scheduling prenatal care, screening tests, and monitoring fetal growth.
Naegele's Rule Formula
The defining relationship is:
With cycle-length adjustment:
where cycle length is the average number of days from the first day of one period to the first day of the next. Rearranged to solve for the other two variables:
Conception = LMP + (cycle − 14) = EDD − 266
Gestational age (GA) at any reference date is:
GA (weeks) = floor(GA_days / 7) · GA (days) = GA_days mod 7
Naegele's Rule Variables
| Symbol | Meaning | Notes |
|---|---|---|
| LMP | Last menstrual period | First day of bleeding; reference point for the rule |
| EDD | Estimated due date | The midpoint of the delivery distribution |
| GA | Gestational age | Weeks + days since LMP; the standard clinical measure |
| Cycle | Menstrual cycle length | Adjusts for ovulation timing; 28 days is the assumption |
| Conception | Estimated fertilisation date | Approximately LMP + 14 days (adjusted for cycle) |
| Term | Full-term window | 39w 0d – 40w 6d per ACOG |
| Post-term | Post-term pregnancy | ≥ 42w 0d; associated with increased risk |
How Each Variable Affects the Due Date
The rule is linear in both LMP and cycle length. A one-day shift in LMP shifts the due date by exactly one day. A one-day change in cycle length shifts the due date by one day as well — because if ovulation is a day later, the pregnancy began a day later and the due date moves correspondingly. In practice, cycle length matters most for women whose cycles are consistently longer or shorter than 28 days: a woman with a 35-day cycle has an EDD about 7 days later than the standard rule would suggest, and a woman with a 21-day cycle has an EDD about 7 days earlier.
Age, parity, ethnicity, and previous pregnancy history also influence actual delivery timing, but they are not part of Naegele's rule — the formula gives a population midpoint, and individual delivery dates vary widely around it. In the largest analysis to date, only about 4% of spontaneous births occurred on the exact EDD; roughly 60% occurred within one week either side; and 90% within two weeks. That is why clinicians describe the due date as a "due window" rather than a date, and why induction decisions are typically made based on gestational age and clinical indication, not the EDD alone.
Pregnancy Due Date Examples
EDD = Jan 5 + 280 days = October 12, 2026
Adjustment = 35 − 28 = +7 days
EDD = Jan 5 + 280 + 7 = October 19, 2026
Adjustment = 21 − 28 = −7 days
EDD = Jan 5 + 280 − 7 = October 5, 2026
LMP = Oct 12 − 280 days = January 5, 2026
Conception = Jan 5 + (28 − 14) = Jan 5 + 14 days = January 19, 2026
Days since LMP = 90 days
GA = 90 / 7 = 12 weeks 6 days = 12w 6d (first trimester)
Frequently Asked Questions
Naegele's rule gives the midpoint of a wide distribution, not a precise prediction. Only about 4% of babies are born on their exact due date, about 60% within one week either side, and about 90% within two weeks. Its accuracy depends heavily on knowing the LMP date and the typical cycle length — irregular cycles, recent hormonal contraception, and uncertain LMP dates all reduce accuracy. First-trimester ultrasound (crown–rump length at 7–14 weeks) estimates the due date to ±5–7 days and is more accurate than any LMP-based rule when both are available. ACOG recommends confirming the EDD by early ultrasound whenever possible.
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Pregnancy actually begins at ovulation (or fertilisation), not at LMP — the two weeks between LMP and ovulation are counted as part of the 280 days even though you were not pregnant yet. If your cycle is longer than 28 days, ovulation occurs later, so the pregnancy starts later and the due date is pushed later by the same number of days. A 35-day cycle gives a due date 7 days later than the standard rule; a 21-day cycle gives a due date 7 days earlier. This is why the cycle-length adjustment matters, especially for women with regularly long or short cycles.
The American College of Obstetricians and Gynecologists (ACOG) redefined term pregnancy in 2013 into four categories: early term (37w 0d – 38w 6d), full term (39w 0d – 40w 6d), late term (41w 0d – 41w 6d), and post term (42w 0d and beyond). Full term — 39 to 40 weeks — is considered the optimal window because outcomes for both mother and baby are best there. Babies born before 39 weeks have higher rates of respiratory issues and NICU admission, and babies born after 41 weeks have higher rates of stillbirth and meconium aspiration. This is why elective deliveries before 39 weeks are generally avoided.
Yes. If you know the conception date — either from tracking ovulation, from IVF timing, or from a single known intercourse date — the due date is conception + 266 days (38 weeks). This is because the standard 280-day count from LMP includes the roughly 14 days between LMP and ovulation that are not part of the pregnancy itself. When conception is known precisely (as in IVF), 266 days from fertilisation (or 261 days from a day-5 embryo transfer) gives a more accurate estimate than the LMP rule. This calculator's "Solve for Conception" mode lets you work backwards from EDD to conception date, or forward from a known conception to EDD.
If you don't know your LMP date or your cycles are irregular, an early ultrasound is the most reliable way to date the pregnancy. A crown–rump length measurement between 7 and 14 weeks is accurate to within 5–7 days, and it is the preferred method when the LMP is unknown or unreliable. Second-trimester ultrasound (14–28 weeks) is accurate to ±10–14 days and is still useful, but less precise than first-trimester. Third-trimester ultrasound should not be used for dating because normal biological variation is larger than the measurement error. If you have no LMP and cannot get an early ultrasound, an approximate dating by fundal height or later ultrasound is better than nothing, but the resulting EDD will be less accurate.
Gestational age is counted from LMP because, historically, that was the only reliably observable event — ovulation and fertilisation are invisible without tracking or imaging. The convention stuck, and it has the practical benefit of adding 2 weeks to the actual fetal age in a way that is consistent across all pregnancies. So when a clinician says "you are 20 weeks pregnant," they mean 20 weeks since your LMP, which corresponds to about 18 weeks of actual fetal development. This is why the gestational age at birth (40 weeks) is roughly 2 weeks longer than the actual time from conception (about 38 weeks). The convention matters when interpreting research and clinical guidance, but for day-to-day use, the two are just different starting points.
Yes, but the adjustment is less reliable with irregular cycles because there is no single "typical" cycle length to apply. If your cycles vary by more than about 8 days from month to month (for example 26 to 34 days), an average is used but the actual ovulation date for that particular cycle could be anywhere in the range. For women with irregular cycles — commonly seen in PCOS, thyroid disorders, or perimenopause — an early ultrasound is the preferred dating method because it directly measures the pregnancy rather than inferring it from a cycle length that may not apply. If you must use the formula, use the average of the last three cycles.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| Due date | Estimated delivery date | |
| LMP | Last menstrual period date |
What it means
Naegele’s rule estimates the due date for a pregnancy based on the first day of the last menstrual period (LMP). By adding 280 days (40 weeks) to the LMP, it gives an approximate delivery date. It assumes a regular 28‑day cycle and ovulation on day 14. While simple and widely used, it has limitations: cycle variations affect accuracy. In modern practice, ultrasound dating is more precise. It is used in prenatal care to monitor fetal growth, plan tests, and manage pregnancy. Understanding this rule is essential for expectant parents and healthcare providers to track pregnancy milestones.
Worked example
Pregnancy Due Date (Naegele's Rule) – Two Detailed Examples
Real‑World| Parameter | Value |
|---|---|
| LMP | Jan 1 |
| Parameter | Value |
|---|---|
| LMP | Jun 1 |
Common mistakes
- Naegele’s rule: Due date = Last menstrual period (LMP) + 280 days.
- Assumes: A 28‑day cycle with ovulation at day 14 – not all women have regular cycles.
- Adjustment: If cycles are longer or shorter, adjust the due date accordingly.
- Calculations: Can also be done by subtracting 3 months from LMP and adding 7 days.
- Ultrasound: Early ultrasound dating is more accurate – this is a rough estimate.
Applications
Naegele's rule estimates the due date of a pregnancy by adding 280 days (40 weeks) to the first day of the last menstrual period. This is the standard method used in obstetrics for pregnancy dating, though it assumes a regular 28‑day cycle and ovulation on day 14. Obstetricians and midwives use it to establish an expected delivery date, to schedule prenatal visits, and to guide pregnancy management. By knowing the due date, healthcare providers can monitor fetal growth, plan delivery interventions, and reassure expectant parents. While not always exact, it is a practical starting point for pregnancy care. Understanding Naegele's rule is essential for maternity care and pregnancy planning.
- Establishment of gestational age and expected delivery date
- Planning of prenatal care and screening tests
- Fetal growth monitoring and intervention timing
- Maternity leave planning for mothers
- Education for expectant parents and health literacy
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